1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display system such as a display unit used in a personal computer and a driving method therefor.
2. Description of the Prior Art
At present, active matrix type (thin film transistor type) liquid crystal display systems are widely used in a great variety of use, and required to be improved in multi-level half-tone (full color).
As a display type for liquid crystal display system, there is so-called twisted nematic display type (thereinafter referred to as "vertical electric field type") wherein liquid crystal is filled between two substrates facing to each other having a flat display electrode on each of the facing surfaces of the substrates, electric field nearly normal to the substrate surface being applied to the liquid crystal to drive. The liquid crystal display system of this type has been already in the market.
On the other hand, another new different type has been proposed, for example, in Japanese Patent Publication No. 63-21907 (1988), wherein a pair of electrodes for applying electric field to liquid crystal are formed on an identical substrate, liquid crystal being driven by applying the electric field to the liquid crystal in the direction approximately parallel to the surfaces of substrate (thereinafter referred to as "parallel electric field type").
In order to produce multi-level halftone display in a thin transistor type liquid crystal display system, the voltage output from the circuit for supplying image signal to signal electrodes, that is, a signal driver IC, needs to have a multi-level output value corresponding to the number of the multi-level halftone. In a case of producing, for example, 16 levels of halftone, the signal driver IC needs to be capable of supplying 16.times.2 (binary values of positive and negative are required for each halftone, since liquid crystal needs to be driven with alternating current.)=32 values of output voltages. Since the signal driver IC has operation amplifiers in the output stages each in order to be capable of supplying sufficient current, it is necessary to provide 32 operation amplifiers in the above case. The smaller the operation amplifier in the output stage can be made and consequently the smaller the signal driver IC can be made, the lower the absolute maximum supply is. Although the productivity of the signal driver IC may be improved and the size of the outer frame portion of the display system may be made small by means of deceasing the size of the signal driver IC, the maximum output voltage for the signal voltage needs to be decreased.
On the other hand, in the parallel electric field type the voltage is applied to the liquid crystal layer with a pair of non-transparent line-shaped electrodes formed on an identical substrate although in the vertical electric field type described above the voltage is applied to the liquid crystal layer with a pair of transparent flat-shaped electrodes facing to each other. Consequently, in the parallel electric field type the opening ratio becomes small. In this reason, since the distance between the two electrodes cannot be so small, the distance between the two electrodes is larger than and the magnitude of electric field in the parallel electric field type is weaker than those in the vertical electric field type. In order to produce the same magnitude of the strength of electric field, therefore, the former needs to apply higher voltage between the electrodes than the latter.
An object of the present invention is to provide an active matrix type liquid crystal display system of parallel electric field type and a driving method thereof in which the display system is operable with a practically sufficiently low drive voltage of signal side drive circuit. Another object of the present invention is to provide an active matrix type liquid crystal display system of parallel electric field type and a driving method thereof in which cross-talk, especially, lateral smear does not appear and high image quality can be attained.